The actions of 5-hydroxytryptamine in the rabbit ciliary ganglion

Intracellular recordings were made from ciliary ganglion neurones of the rabbit in vitro. 5-Hydroxytryptamine (5-HT) produced a rapid depolarization in 67% of the neurones examined. The 5-HT-induced depolarization persisted in solutions containing zero (1-100 microM), or on the amplitude and the duration of the current used for ionophoresis. The 5-HT-induced depolarization was associated with a marked fall of input resistance and reversed in polarity at about -10 mV. The 5-HT-induced response was reversibly abolished in sodium-free solution. A marked desensitization to this action of 5-HT was observed. After the 5-HT depolarization passed off due to desensitization, 5-HT (1 microM-1 mM) reversibly depressed the amplitude of cholinergic excitatory postsynaptic potentials (EPSPs) in 91% of the neurones tested in a dose-dependent manner, whereas 5-HT did not detectably affect the depolarization induced by ionophoresis of acetylcholine (ACh). The frequency of spontaneous miniature EPSPs (in 20 mM potassium solution) was decreased by 5-HT (30-100 microM). These results demonstrate that 5-HT exerts two separate actions in the rabbit parasympathetic ciliary ganglion. One is a depolarization of ganglion cells due to an increase in membrane permeability to sodium and presumably potassium ions; the other is a depression of ACh release from presynaptic fibres. Further pharmacological characteristics of these actions of 5-HT were investigated and discussed.